The science of analytical chemistry and particularly simple to use dry reagent test devices using analytical chemistry principles has made dramatic progress over the past several decades. At one time such devices simply comprised a piece of filter paper impregnated with the dried residue of a pH indicator or a relatively simple test reagent composition. Devices such as these usually gave an indication of the presence or absence of a substance or a gross condition of the fluid being analyzed, such as, for example, the use of litmus paper to determine if the fluid is acidic or basic. Now such devices are much more complex in structure and composition and can give answers which are as precise, specific and sensitive as those obtained using laboratory procedures and conditions. Moreover, such devices can quite often be used without accompanying instrumentation which permit their use in the field or "on-site" to give nearly instant answers. This obviously eliminates the need for preserving sample integrity, simplifies record keeping and allows the user to take rapid corrective measures.
Dry reagent test devices commonly consist of a bibulous or porous paper or polymeric matrix incorporating a reagent composition which reacts with the substance being determined. The first of such systems were the reagent strip or dip and read type devices which came into widespread use with the introduction of urine screening diagnostic tests during the late fifties and early sixties. Such test devices usually comprise a flat absorbent paper or polymeric matrix pad incorporated with a chemical or biochemical reagent which reacts specifically with the substance being detected (the analyte) to give a measurable response. This measurable response commonly comprises a color which is read visually but may be measured instrumentally to give more accurate and consistent readings. The amount of color is then translated into concentration of analyte in the fluid being tested by either using standard color blocks or algorithms. The reagent pad is often attached to a plastic handle for ease of support and use to become what is known in the art as a reagent strip test device.
Another type of dry reagent test device is the reagent impregnated bibulous or porous matrix which is enclosed or encased in a fluid impervious sheath or covering, usually plastic, which restricts and defines the flow of fluid being tested to an assigned opening, usually located at an end portion of the sheath. In use, this type device is contacted with the fluid being tested such that the opening is exposed to the fluid which wicks up or into the bibulous matrix by capillary action (or is pulled or pushed through the porous matrix), wherein the analyte or a conversion product thereof in the fluid reacts with the reagent to form a localized reaction product giving a visual response as the fluid moves through the matrix. This type device is known as a sheath encased reagent impregnated matrix or SERIM type test device.
The reagent system which is used to impregnate the absorbent pad of the reagent strip test device or the matrix of the SERIM device is more often than not a combination or mixture of chemicals, biochemicals or immunochemicals. The more sophisticated and complicated the reagent system, the more difficult it is to incorporate into the absorbent pad. For ease of formulating and manufacturing, the ideal dry reagent test device comprises a relatively simple chemical mixture incorporated into a single absorbent pad or matrix. When reagent incompatibility is encountered, it is common practice to attempt separation of the various components either chemically or physically. One means commonly utilized is to separate the various components in a single matrix using selective solvent impregnation techniques. Another means is to encapsulate one reagent so that it will not react with the others present in the system until it comes in contact with the fluid being tested.
More recently, it has become the practice of reagent strip or SERIM device formulating scientists to separate the reagents using multilayer reagent strip devices in which the various components are retained in separate layers of the matrix until the test device is utilized. Such multilayer devices have several advantages. In addition to accomplishing the separation of reagents for stability purposes, such matrices can be utilized to pretreat or concentrate the analyte or fluid being tested or to remove or complex an undesirable component or constituent in the sample fluid. It is common practice in the reagent strip art to utilize multilayer matrices; however, such matrices must meet the rather strict requirement that the layers be uniformly bound to each other and that fluid must flow evenly and freely throughout the device. In this regard, to date, most commercial multilayer test devices utilize a series of gel layers such as in film type devices wherein the layers are constructed by pouring one layer on top of the other and using the natural adhesiveness of the gel material for layer attachment.